Spinal fusion is a procedure that involves joining two or more adjacent vertebrae with a bone fixation device to restrict movement of the vertebra with respect to one another. For a number of known reasons, spinal fixation devices are used in spine surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the fixation element to various bone fixation elements, such as hooks, bolts, wires, screws, etc. The fixation elements can have a predetermined contour, and once installed, the fixation element holds the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
Recently, there has been a movement away from the use of rigid fixation elements and more towards dynamic or flexible fixation elements. Dynamic fixation elements are desirable because they reduce the stress inherent in a rigid fixation system, and absorb shock, for example, in the extension and compression of the spine. In addition, the removal of bone structure, such as facet joints or laminae, result in instabilities of the motion segments of the spine. Consequently, a fixation system should stabilize the motion segment in anteroposterior translation as well as in axial rotation. Both motion patterns result in shear stress within the rods of the fixation system. This is especially important in elderly patients, where the bone quality is sometimes compromised, becoming sclerotic or osteoporotic.
Existing dynamic fixation systems incorporate mechanisms to absorb shock, for example, in extension and/or compression of the spine, but existing dynamic fixation systems generally lack sufficient strength and/or constraints to overcome expected shear stresses as a result of anteroposterior translation and/or axial rotation of the spine.
Thus, it is desirable to have a dynamic fixation system which can withstand the expected stresses including shear stresses caused by anteroposterior translation and/or axial rotation.